Plagioclase ultraphyric basalts in Iceland: the mush of the rift

Glassy, plagioclase ultraphyric basalts from six locations in Iceland have bimodal phenocryst size distributions where microphenocrysts (ol+plg +/- cp x +/- mt) are in equilibrium with the matrix glass, but macrophenocrysts (o l+plg +/- cpx) are too primitive to be so. Matrix glass compositions are si milar to those of other rift zone glasses from Iceland, and oxygen isotope variations suggest they interacted with the Icelandic crust. A lack of nega tive Eu-anomalies in matrix glasses precludes large amounts of plagioclase crystallisation from their parental liquids. Compositions of glass inclusio ns in plagioclase and olivine macrophenocrysts indicate that parental magma compositions of the macrophenocryst assemblage are similar to those of pri mitive, Icelandic rift zone glasses. Temperatures for plagioclase macrophen ocryst crystallisation obtained from Linkam(R) heating stage experiments, a nd from glass inclusion compositions corrected for post-entrapment crystall isation, give temperatures up to 1260 degrees C, corresponding to crystalli sation at middle to deep crustal levels. Temperature differences of less th an 100 degrees C between plagioclase-hosted glass inclusions before and aft er post-entrapment plagioclase crystallisation show that the macrophenocrys ts must have been kept at elevated temperatures prior to incorporation in t heir present host magmas. We suggest that the macrophenocrysts of the plagi oclase ultraphyric basalts accumulated in crystal mush bodies underneath th e rift zone and were picked up by their present hosts during a rifting even t with increased magma supply from the mantle. (C) 2000 Elsevier Science B. V. All rights reserved.